The Barack Obama Presidential Center in Chicago could require a $1.5 billion endowment, its architects say, three times what was raised for the George W. Bush Presidential Center in Dallas.

Husband-and-wife architectural team Tod Williams and Billie Tsien noted that it will be difficult to raise such a huge sum because Obama scrupulously declined to do much fundraising while he was still in office.

The Obama Center is due to be so expensive because it will require the construction of both a presidential library and a museum about the lives of Barack and Michelle Obama. And federal requirements now stipulate that former presidents must have larger endowments to pay for annual operating costs at the libraries.

“It won’t be easy,” Williams said. “It’s not just about preserving the past. It’s about the future.”

The actual buildings were slated to cost $200 million. “But I told them it will cost $300 million,” Williams said.

Williams and Tsien spoke about the project with architectural critic Paul Goldberger on Wednesday, at the annual benefit for East Hampton’s LongHouse Reserve.

The event was held in the David Rubenstein Atrium at Lincoln Center, which the duo designed. They’ve also designed the Barnes Foundation in Philadelphia, the Phoenix Art Museum and the Logan Center for the Arts in Chicago.